A time of flight (TOF) method for measuring a distance based on flight time in which light travels to and from an object, using a solid state imaging element (image sensor) is known among a plurality of methods for detecting an object.
FIG. 24 is a plan view illustrating a part of a unit pixel 900 of a solid state imaging element that uses the TOF method. The unit pixel 900 is a conventional art disclosed in Patent Literature 1. The unit pixel 900 includes a plurality of light receiving devices 100. In this embodiment, the unit pixel 900 includes four light receiving devices 100 arranged in a matrix.
The unit pixel 900 includes the four light receiving devices 100 arranged in the matrix. The light receiving devices 100 includes: a photoelectric conversion element 104 formed above a semiconductor substrate; four photoelectron distributing units 106; and two photoelectron discharging unit 108. The photoelectric conversion element 104 has a photo gate structure with electrodes formed above the semiconductor substrate through an insulator. The photoelectric conversion element 104 is a photodiode that detects light, and generates photoelectron (negative charge), that is, converts the detected light into photoelectrons. A gate drive signal for driving the photoelectric conversion element 104 is input to a photo gate from the gate drive circuit. In addition, the photoelectron distributing unit 106 includes: a first transfer unit 112; a photoelectron holding unit 114; a second transfer unit 116; and a floating diffusion layer 118. The first transfer unit 112 transfers the photoelectron generated by the photoelectric conversion element 104 to the photoelectron holding unit 114.